The operating current of recent CPUs (Central Processing Unit) has been increasing due to high integration as necessitated by their increasing processing capabilities. Moreover, in highly integrated CPUs, leakage current due to poor insulation caused by finer circuit patterns and so on has also been increasing, causing an increase in consumption current in the CPUs. Therefore, in recent years, semiconductor devices that consume a device driving power exceeding 100 amperes have been developed.
Since such high power devices (semiconductor devices that consume a large current) need large numbers of interconnections for device driving power (VDD), ground (GND), and input and output signals, they are generally mounted in a package style called BGA (Ball Grid Array) or LGA (Land Grid Array).
In the semiconductor device employing BGA or LGA, a large number of electrode pads (device electrodes) are disposed in a matrix at the pitch of about 1 mm on the back face of the device substrate (opposite the mounting substrate).
Generally, semiconductor devices go through discrimination tests to see if each semiconductor is a good product or a defective product, and also they go through a test called a burn-in to find initial failures. In a burn-in, the semiconductor devices are placed under a heavier load than usual.
In a burn-in test, semiconductors are mounted on burn-in sockets that are arranged on a burn-in substrate. The burn-in substrate is realized by a multi-layer printed board which is prepared by patterning electrode pads on its front face in the same matrix pattern as the device electrodes, wherein the electrode pads are provided with wiring for VDD, GND, and input and output signals.
The burn-in sockets make electrical connections between the device electrodes and the electrode pads of the burn-in substrate with contact pins that are arranged in the same matrix pattern as the device electrodes. To reduce cost, the contact pins of the burn-in sockets are generally provided only for the electrode parts required for burn-ins. However, burn-in sockets are still fairly expensive. The burn-in substrates are also expensive since multi-layer printed boards are used, and accordingly burn-in substrates that mount a plurality of burn-in sockets are very expensive.
Examples of burn-in substrates realized by multi-layer printed boards are disclosed in Japanese Publication for Unexamined Patent Application Nos. 68557/1997 (Tokukaihei 9-68557, published on Mar. 11, 1997) and 221234/2000 (Tokukai 2000-221234, published on Aug. 11, 2000), and Japanese Patent No. 3392783 (published on Jan. 26, 2001).
The burn-in test is carried out to find and remove potential defective products that may cause initial failures with high probability. Therefore, the burn-in test assumes the presence of defective semiconductor devices. In some type of defects, an overcurrent may flow across VDD and GND of the semiconductor device to burn the burn-in substrate or burn-in sockets.
Conventional high-power devices have consumption current of about 30 amperes. Therefore, in order to supply current, one or more conductor layers of a multi-layer printed board realizing the burn-in substrate has been used as the wiring layer for VDD and GND. However, since the thickness of the conductor layer (copper foil) in the printed board is as thin as 35 μm to 70 μm, it has been difficult to supply power to high power devices which consume a large current exceeding 100 amperes. Moreover, due to a current loss which increases as the square of a current value with respect to the conductor resistance, a problem arises where, for example, it is impossible to quickly follow voltage drop, heat generation of wiring conductors, and changes in consumption current as a result of a change in the operation state of the semiconductor devices.
Furthermore, since the maximum current capacity of wiring layers have no margin against the normal consumption current of the semiconductor devices, even a slight overcurrent generated by a device defect may quickly damage the burn-in substrate before the over current protector (OCP) of the power supply units that supply VDD operates.
Since it is almost impossible to repair such burn-out, a part of or the whole of the burn-in substrate becomes unusable when it is damaged by burn-out. Especially, high power devices that consume a large current have a high risk of burn-outs, and any incurred loss will be expensive.